Switched reluctance motors conventionally have poles or teeth on both the stator and the rotor (i.e. they are doubly salient). There are phase windings on the stator but no windings on the rotor. Each pair of diametrically opposite stator poles is connected in series to form one phase of the switched reluctance motor.
Torque is produced by switching current on in each phase winding in a predetermined sequence that is synchronized with the angular position of the rotor, so that a magnetic force of attraction results between the rotor and stator poles that are approaching each other. The current is switched off in each phase at the commutation point before the rotor poles nearest the stator poles of that phase rotate past the aligned position, otherwise the magnetic force of attraction will produce a negative or braking torque. Quick reduction of the phase current to zero helps avoid negative torque. Co-pending application Ser. No. 612,517, now U.S. Pat. No. 4,500,824, issued Feb. 19, 1985, and assigned to the assignee of the present application, discloses a three stage commutation method for a switched reluctance drive.
The torque developed is independent of the direction of current flow. Unidirectional current pulses synchronized with rotor movement can be generated in a converter using a single unidirectional current switching element such as a thyristor or transistor in each phase.
Each time a phase of the switched reluctance motor is switched on by closing a switch in a converter, current flows in the stator winding, providing energy from a dc supply to the motor. The energy drawn from the supply is converted partly into mechanical energy by causing the rotor to rotate towards a minimum reluctance configuration and partly into a magnetic field. When the switch is opened, part of the stored magnetic energy is converted to mechanical output and the remainder of the energy is preferably returned to the dc source.
In prior art switched reluctance motors, efficiency is reduced because of unrecovered energy in the magnetic field. It is known to recover some of this energy by using a bifilar winding to allow current to return to the dc source after the main switching device turns off. The bifilar winding permits this energy recovery without having recourse to alternative circuits that require two switching devices and two freewheel diodes in each phase. However, bifilar windings have the disadvantages of high cost, poor winding space utilization and a doubling in the number of terminal connections.
It is also known to use a single winding with a bipolar power supply. A bipolar supply is undesirable because the available dc voltage cannot be utilized efficiently and because the base drive and other control circuits have to be supplied through isolation transformers, increasing overall costs.